This invention generally relates to an objective lens assembly of variable focal length for use with an optical picture taking apparatus, for example, a photographic still camera, a movie camera or a television camera. More particularly, this invention relates to a varifocal lens assembly of a type wherein the lens speed or the aperture ratio of the lens assembly varies with a change in the focal length.
Current commercially available zoom lens assemblies are generally classified into fixed speed types whose optical systems are designed such that the lens speed or the aperture ratio does not vary even when the focal length thereof is adjusted for zooming with the diaphragm aperture kept unchanged, and variable speed types whose optical systems are designed such that the lens speed or the aperture ratio varies when the focal length thereof is adjusted with the diaphragm aperture kept unchanged. The variable speed type zoom lens assemblies are furthermore classified into two subtypes: compensated types, wherein a mechanical compensator system is built in for compensating for change in lens speed resulting from an change in the focal length, and non-compensated type wherein the lens speed is allowed to vary with change in the focal length without any compensator system employed. Both of these subtypes have their own merits and demerits, however they play a role in the market to accommodate a diversity of customers' demands.
As is well known to those skilled in the art, for a given diaphragm aperture, the variable speed type is generally faster, i.e., has a smaller F-stop number at the shortest focal length setting than at the longest focal length setting. To compensate for the reduction of the lens speed to make the lens speed constant at all values of focal length, means must be provided for stopping down the diaphragm aperture as the focal length is adjusted to the shortest value, and this is currently embodied in the compensated model such as disclosed in, for example, U.S. Pat. No. 4,258,996. In this compensated model, the maximum available performance of the optical system of the zoom lens assembly cannot be enjoyed, particularly when the focal length is adjusted to a wide angle range. It should be noted that the term "wide angle" hereinabove and hereinafter used and the term "tele-angle" which will hereinafter be used in an opposite sense to the term "wide angle," are a relative notation applicable to a particular adjustable range of focal length of a given zoom lens assembly and correspond respectively to the reading on a focal length setting element such as, for example, a zooming ring, approaching the shortest focal length and that approaching the longest focal length. By way of example, for any zoom lens assembly having it focal length adjustable within a range of 28 mm to 55 mm, the focal length setting to a value on the side of the 55 mm position or the longest focal length may be said to be the setting to the "wide angle" range and that to the "tele-angle" range, respectively.
As hereinbefore described, in the variable speed zoom lens assembly of the compensated type, the maximum available performance of the optical system is sacrificed at the wide angle range to tailor the lens speed or aperture ratio to a value given at the longest available focal length setting. However, this compensated model is generally considered convenient in that, since in practice the smallest available aperture reading, that is, F-stop number, on the aperture setting ring is calibrated to a value given at the longest available focal length setting, the photographer need neither readjust the aperture setting ring in an attempt to compensate for reduction in lens speed resulting from the change of the focal length, nor keep in mind the necessity of such a readjustment, and, also, the lens assembly has a consistency with the exposure control system of a photographic camera. On the other hand, although the variable speed zoom lens of non-compensated type, such as disclosed in, for example, Modern Photography, November 1977, Vol. 41, No. 11, pages 164 to 168, is advantageous in that the maximum available performance of the optical system can be fully enjoyed at any focal length setting, not only does the layout in the non-compensated model of the aperture reading on the aperture setting ring in combination with an index marking pose a problem, but also the lens assembly lacks a consistency with the exposure control system.
Specifically, in the variable speed zoom lens assembly of the non-compensated type referred to above, the smallest aperture reading on the aperture setting ring includes two F-stop numbers available generally at the wide angle range and the tele-angle range, respectively, in combination with the index marking including the juxtaposed aperture-indicating indicia associated respectively with the smallest F-stop numbers. So far as the lens assembly disclosed in the Modern Photography, supra, is concerned, it is described that the 43-75 mm zoom lens assembly is an f/3.5 for most of its zoom, i.e., the focal length setting, but becomes f/4.5 at the maximum 75 mm setting, and two juxtaposed aperture-indicated indicia are employed in the form of a line and a dot on the fixed barrel around which the aperture setting ring can be rotated.
For better understanding of this invention, the merits and demerits of each of the compensated and non-compensated models of the variable speed zoom lens assemblies will be hereinafter discussed in detail under separate headings.